toxicity in sediments may be extremely patchy, so that healthy organisms 

 are found adjacent to barren areas. If the latter two hypotheses prove 

 to be true, toxicity in the Illinois River has changed recently from a 

 widespread, chronic problem to a more localized or episodic problem. 

 Reduction of toxicity in surface sediments may reflect recent reductions 

 in ammonia loading from sewage treatment plants in the Chicago area, 

 although it is not clear whether the sources of ammonia in the pore- 

 waters are effluents, the deeper layers of sediments (as described 

 above) , or both. 



We remind the reader that all the toxicity tests we conducted were 

 short-term, acute tests. The fingernail clams, MuscuTium transversum, 

 were exposed to sediment porewater for only 1 hour and then their fil- 

 tering performance was tested in clean dilution water. The water flea, 

 Ceriodaphm'a dubia, was exposed to porewater for just 48 hours. The 

 organisms in the waterways are exposed to contaminants for their entire 

 life spans. In the past, more sensitive tests with fingernail clams 

 have demonstrated toxicity even in downriver sediments, including Peoria 

 Lake and Quiver Lake (Sparks, Sandusky and Paparo 1981). 



In addition to being a problem for the benthic invertebrates that 

 fish feed upon, ammonia in the Illinois Waterway may be a problem for 

 the fish themselves. In 1987, the U.S. Fish and Wildlife Service simu- 

 lated resuspension of bottom sediments by boat- or wind-driven waves by 

 stirring sediments in clean water, allowing the sediment to settle for 

 24 to 48 hours, then exposing larval fathead minnows, Piwephales prome- 

 las, to the water. Water mixed with surface sediments from the Chicago 

 River and the Des Plaines River killed all the fish within 24 hours. 

 Surface sediments from Lake Chautauqua, a bottomland lake and federal 



49 



